1. Field of the Invention
The present invention relates to a squirrel-cage rotor and an electric motor having the squirrel-cage rotor.
2. Description of the Related Art
FIG. 6 is a sectional view showing a laminated core 100 of a rotor according to the related art. The laminated core 100 is provided with a plurality of slots 102 spaced apart from each other in a circumferential direction C (rotational direction), and includes teeth 104 forming a magnetic flux path through the laminated core 100, and the teeth 104 and slots 102 are arranged alternately in the circumferential direction C. The illustrated slots 102 formed in the laminated core 100 are closed slots, whose outer circumferential edges are closed by the outer circumference 100a of the laminated core 100. The slots 102 of this type are entirely surrounded by the laminated core 100.
FIG. 7 is a partially enlarged view illustrating one slot 102 of the laminated core 100 shown in FIG. 6 and its periphery. When an electric motor with a rotor which includes the laminated core 100 is rotated at high speed, stress concentration occurs in areas close to a slot bottom portion 102a and a slot top portion 102b. In the case where an inner circumferential edge 106 and an outer circumferential edge 108 of the slot 102 are defined by a simple arc shape, the thickness of the laminated core 100 in a radial direction R is sharply reduced at the slot bottom portion 102a and the slot top portion 102b. This results in considerable stress concentration. In FIG. 7, broken lines show areas Q1 and Q2 where stress concentration occurs.
Japanese Patent JP-A-6-253511 discloses a rotor provided with elongated slots extending in the circumferential direction, in order to reduce the maximum stress generated in the circumferential edges of the slots. FIG. 8 is a partially enlarged view illustrating two adjoining slots 202 and their peripheries of a laminated core 200 disclosed in JP-A-6-253511. As shown in the drawing, the slots 202 according to this related art have a characteristic sectional shape whose size defined in the circumferential direction C is larger than that defined in the radial direction R.
However, if the slots 202 have an elongated shape in this manner, bridge portions 206 defined between the slots 202 and the outer circumference 204 of the laminated core 200 also have an elongated shape in the circumferential direction C. This increases the centrifugal force acting on the bridge portion 206. The bridge portion 206 receives centrifugal force acting on a conductor (not shown) accommodated in the slot 202. The centrifugal force acting on the conductor is also increased when the slot 202 has an elongated shape in the circumferential direction. As a result, a tensile force, which pulls the bridge portion 206 toward the outside in the radial direction R, is increased. This results in increased stress generated in areas Q5 and Q6 at positions near the respective end portions of the slot 202. In addition, stress concentration tends to occur in the areas Q5 and Q6 where the curvature of the slot 202 is sharply changed. Therefore, unless the areas Q5 and Q6 have sufficient strength, the bridge portion 206 may possibly fracture at the areas Q5 and Q6 and break off from the laminated core 200. On the inner side of the slot 202 in the radial direction R, stress concentration similarly occurs at areas Q3 and Q4 where the curvature of the slot 202 is sharply changed. In addition, since the areas Q3 and Q4 receives centrifugal force acting on a tooth 212 extending between the slots 202, increased stress occurs at the areas Q3 and Q4. Therefore, unless the areas Q3 and Q4 have sufficient strength, this may cause the laminated core 200 to fracture. In the rotor according to this related art, the rotational speed of the electric motor is limited, depending on the strength of the areas Q3 to Q6 where stress concentration occurs.
Further, according to this related art, side portions 208 and 210 of the slot 202 extending along the radial direction R have a protruding shape which protrudes toward an opposite side portion 210 of another slot 202 arranged next thereto in the circumferential direction C. As a result, a tooth 212 formed between the adjoining slots 202 has a narrower portion 214 whose width becomes locally small. Since the narrower portion 214 limits the amount of magnetic flux passing through the laminated core 200, the amount of torque generated by the electric motor is reduced.
Accordingly, there is a need for a rotor which reduces stress concentration in a laminated core, allowing for increased rotational speed, and for an electric motor having such a rotor.